An ancient law of Canterlot states that each species present within the city must have an adequate diplomatic representation. There are no exemptions from it, even as far as interdimensionals aliens are concerned. As such, Anonymous the Human must pick up the mantle of an ambassador for humanity, even if he is the sole specimen. Even if he doesn't want to.
But it's not all bad news for him. After all, with such a responsibility also come some privileges. Personal guard detail, generous private mansion, all expenses covered by the crown. And of course, diplomatic immunity. Only that, thanks to the pony magic, the word immunity is taken much more seriously in Equestria.
Proofread by Havana